The bottom image shows particles being sucked into the donut shaped microfluidic vortex, resulting by the hologram-laser approach. The nanoparticles concentrate into a tight clump, ideal for nanoassembly. (Source: Purdue University)

Laser and hologram method could also be useful for positioning molecules in labs-on-a-chip

A breakthrough from Purdue University offers up a new technique that may be extremely useful in such nanoassembly. The new method uses lasers and holograms to position multiple nanoparticles within seconds, much faster than previously possible. The method could also be very helpful to labs on the chip, which need to direct tiny molecules to tiny test locations.

The name for the new process -- rapid electrokinetic patterning -- is a bit overwhelming, but it’s an incredibly promising technology according to researcher Stuart J. Williams. Mr. Williams, who is working with doctoral student Aloke Kumar and Steven T. Wereley, an associate professor of mechanical engineering, states, "It's potentially a very versatile tool."

His method uses two parallel electrodes made of indium tin oxide. Indium tin oxide is a transparent conductive material. A tiny gap of 50 micrometers, or approximately the diameter of a human hair, separates the parallel plates. By simultaneously apply a current to the plates and shining a laser through them, holograms can be created that position molecules travelling between the plates.

In a test, the team position fluorescent beads. Mr. Kumar describes, "We send holograms of various patterns through this and, because they are holograms, we can create different shapes, such as straight lines or L patterns."

By creating desirable patterns, molecules, which are drawn to the hologram's pattern between the plates, could be assembled into features for nanomachines. This could enable a miniature assembly line which produces nanomachines piece-wise.

The system is very flexible. Says Mr. Kumar, "It's a very dynamic system, so we can change this pattern quickly."

The process works due to heating. The lasers heat the fluid between the plates slightly. By applying a current, the heated fluid begins to swirl, much like convective currents, except on a microscopic basis. The resulting "microfluidic vortex" sucks molecules in. By altering the laser and the current, different vortex shapes can be attained.

Mr. Williams describes, "You could take one particle, a hundred particles or a thousand particles and move them anywhere you want in any shape that you want. If you have particles of two different types, you can sort one group out and keep the other behind. It's a versatile tool."

The method could result in both assembly and lab-on-a-chip designs far better than the current methods at moving and congregating molecules. Currently one popular method is optical trapping, which uses a beam of light to move particles. This method, however, is too slow compared to the new method, as it moves too few particles. The other alternative -- dielectrophoresis -- is comparably fast, but its patterns cannot be changed after the current is switched on.

The device could help further the lab-on-a-chip revolution in the medical industry, which promises better detection of cancer and many other diseases. Describes Mr. Williams, "If you want to pattern individual particles on a massive scale using electrokinetic methods as precisely as we are doing it, it could take hours to days, where we are doing it in seconds. For example, a single drop of blood contains millions of red blood cells and countless molecules. You always want to have the smallest sample possible so you don't generate waste and you don't have to use as many chemicals for processing the sample. You want to have a very efficient high throughput type of device."

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